As is known, a two-shaft gas turbine is a machine consisting of a compressor, one or more combustion chambers, and two turbine wheels with one or more stages; one turbine wheel is connected by a shaft to the compressor, while the other wheel is connected to the load by the second shaft.
Air taken from the external environment is fed to the compressor to be pressurized. The compressor can be provided with suitable vent valves, also known as bleed valves, which discharge some of the compressed air to the atmosphere.
The pressurized air passes over the outside of the combustion chamber jackets, thus cooling them, and then reaches a set of burners which have the function of mixing the air and the fuel gas (obtained from external pipes), thus providing a gas-air mixture for burning. The pre-mixing of the air with the gas enables the local temperature to be contained in the following primary combustion region, thus limiting the formation of pollutants such as nitrogen oxides.
The combustion reaction takes place inside the jackets, where the temperature and consequently the enthalpy of the gases are increased.
The gas at high temperature and high pressure then passes through suitable pipes to the different stages of the turbine, which converts the enthalpy of the gas to mechanical energy available to a user.
It is known that, in order to obtain the maximum efficiency of any given gas turbine, the temperature of the gas at the inlet to the first turbine wheel, referred to below as the temperature TFire, must be as high as possible; however, the maximum temperatures that can be reached during the use of the turbine are limited by the strength of the materials used.
It is also known that, in order to obtain low emission of pollutants, the fuel-air ratio (abbreviated to F/A in the following text) must be suitably controlled; however, the acceptable values of F/A are limited by problems of loss of ignition in the gas turbine or the generation of pressure pulsations in the combustion chamber.
In practice, there is a requirement to design a thermodynamic cycle for the two-shaft gas turbine which will yield high efficiency combined with low emission of pollutants.
However, the nominal thermodynamic cycle of a gas turbine is modified in practical applications by disturbance factors such as:                variations of environmental conditions (pressure, temperature and humidity);        variations of pressure drops in the inlet air intake pipes;        variations of the pressure drops in the exhaust gas discharge pipes;        variations of the speed of the low pressure shaft (connected to the user).        
If due allowance is not made for these disturbance factors, they may have the following effects:                failure to achieve the maximum temperature TFire at the inlet of the first turbine wheel in full load conditions (with consequent reduction of the thermodynamic performance of the turbine);        exceeding of the maximum temperature TFire at the inlet of the first turbine wheel in full load conditions, with consequent reduction of the maintenance interval for the turbine;        failure to achieve the correct fuel-air ratio F/A in the combustion chamber with a consequent increase in the emission of pollutants such as nitrogen oxides (also abbreviated to NOx in the following text) and carbon monoxide, and the appearance of dangerous pressure pulsations in the combustion chamber or loss of ignition in the combustion chamber.        